The present invention relates to a method of forming a high-quality thin film of a metal or metal compound by vapor growth.
As one conventional vapor growth apparatus, a diffusion furnace type low-pressure vapor growth apparatus is known wherein a plurality of substrates is aligned along the longitudinal direction of a reaction tube, and a thin film is deposited by vapor growth on each of the substrates. The reaction tube of this apparatus comprises quartz. This quartz reaction tube can be effectively used to form a polycrystalline silicon film and a silicon nitride film since quartz has a high purity and the tube can easily be cleaned. However, the quartz reaction tube has the following problem when a metal or metal compound film is formed.
For example, when a tungsten (W) film is formed on a substrate surface by using a gas mixture of tungsten hexafluoride (WF.sub.6) and hydrogen as a feed gas, the tungsten film is deposited on the substrate surface and the inner wall surface of the quartz reaction tube. However, adhesion between the quartz and tungsten is poor. As the number of growth operations is increased, the tungsten film deposited on the inner wall of the quartz reaction tube is separated. Pieces of the separated tungsten film adhere to the substrate surface. As a result, granular defects occur on the substrate surface. In addition to this disadvantage, the dissociation speed or rate of WF.sub.6 at the quartz surface is lower than that at the tungsten film surface. Thus, the concentration of WF.sub.6 reaching the substrate surface changes in accordance with the area of the tungsten film formed on the inner surface of the quartz tube. For this reason, a tungsten film having a desired film thickness cannot be formed on the substrate surface with good reproducibility.